Well I did two continents with just a 28mm and 50mm prime. Wouldn't really recommend it though, kind of a pain in the ass, but by the end of that 3 months I was literally seeing everything as it would look through the 50mm lens.
I'd love to take something longer like a 120-300mm f2.8 but the weight combined with size and the fact that its not as good with general closeups as the 70-200mm with a 2*TC means it would probably be best left behind unless I were specifically touring wildlife spots alone.
(Ps I'd also probably smuggle a point and shoot or Olympus OMD + lens for some general snaps and wider angle work)
Oh man tough call but probably the fast mid range zoom like 17-55 2.8 on dx or 24-70 on fx. If I could take 2 it would prob be a 16-35 or 14-24 paired with a 50 1.8g on fx.
My current 3 lens setup is 10-20, 35 1.8, 80-200 2.8. I took that around Europe and I may take it with me all over north america pretty much equivent of what I would want on fx but for dx.